Fix shared options between instances of DataTables

[Co-qn]

Summary

When using multiple data tables in a HTML, it seems that options for each instances are shared.
I discover it is because of the defaultConfig object is broken changed when creating DataTable instances.

Example

var myData = {
    "headings": [
        "Name",
        "Company",
        "Ext.",
        "Start Date",
        "Email",
        "Phone No."
    ],
    "data": [
        [
            "Hedwig F. Nguyen",
            "Arcu Vel Foundation",
            "9875",
            "03/27/2017",
            "nunc.ullamcorper@metusvitae.com",
            "070 8206 9605"
        ],
        [
            "Genevieve U. Watts",
            "Eget Incorporated",
            "9557",
            "07/18/2017",
            "Nullam.vitae@egestas.edu",
            "0800 106980"
        ],
    ]
};

var table1 = new DataTable("#dt1", { data: myData, perPage: 25 });
var table2 = new DataTable("#dt2", { });

Expected

• table1 has set default perPage = 25.
• table2 has set default perPage = 10(default).
• table1 has 2 records.
• table2 has no records.

Actual(before fixed by this PR)

• table1 has set default perPage = 25.
• table2 has set default perPage = 25.
• table1 has 2 records.
• table2 has 2 records.

[johanneswilm]

@Co-qn Have you checked whether this is the only thing that is apparently leaking into the global namespace?
I wonder if it would be an idea to refactor the entire thing as ES2015+ modules. That should stop leakage (as well as make the sources more readable by spreading them out into several files).

[Mobius1]

@johanneswilm - The ES2015+ module route was something I was considering. Maybe it's time to put it in to action as the library seems to be fairly popular. I'll look into refactoring after the weekend.